The determination of physical parameters of layers making up, e.g., a laminated or layered structure is very important, since many modern technologies use multi-layered materials for various functions. For example, in magnetic disks a thin magnetic underlayer deposited between a supporting substrate layer and a top protective layer (e.g., a diamond-like-carbon (DLC) layer) is used to store data. Precise characterization of the magnetic underlayer is essential to further advances in the field of magnetic storage. It is especially important to determine such physical parameters as thickness t.sub.u, index of refraction n.sub.u and extinction coefficient k.sub.u of the magnetic underlayer. This presents considerable difficulty for many reasons. First, the underlayer is covered by the top layer and hence can not be measured directly by a non-destructive technique. Second, the magnetic underlayer, if exposed, will undergo oxidation and hence a direct measurements of its physical parameters will be flawed and will not reflect the actual physical parameters of the magnetic underlayer covered by the top layer. Third, the thickness of the top layers in this application as well as in many other applications where an underlayer is covered by a top layer is in the range of a few hundred Angstroms or even tens of Angstroms. In this range, typical optical measurements are not very reliable and hence the determination of physical parameters of the underlayer through such a thin top layer presents a challenge to optical methods.
Various prior art techniques exist for examining top thin films or layers. U.S. Pat. No. 3,601,492 to Reichert employs a standard interference technique for measuring film thickness based on observing the interference between the light reflected from the top and bottom surfaces of the thin film. Greenberg et al. teaches in U.S. Pat. No. 5,042,949 that film thickness can be determined by examining the interference pattern and reflectance data from a reflectance pattern, respectively to determine film thickness profile. Still another approach to determining thin film thickness is taught by Hattori et al. in U.S. Pat. No. 5,371,596. Here, the light from a light source is modulated to produce a modulated interference light. This modulated light is reflected from the thin film and used by a number of photodetectors to derive film thickness.
In U.S. Pat. No. 4,999,509 Wada et al. describe a how to measure thicknesses of several films using a reflectance measuring device.
Unfortunately, the above prior art approaches yield less and less satisfactory results for the thin film parameters with decreasing film thickness due to poor signal-to-noise ratios. Moreover, most of these techniques are for determining top films and are not easily adaptable to measuring the physical properties of underlayers.
There are various other prior art approaches to measuring thin film thickness and other physical parameters. However, most of these are complicated and not capable of providing the desired levels of accuracy. Moreover, none of the prior art techniques can be adapted for high-precision measurements of sandwiched films or underlayers.
Hence, there is a pressing need to develop an approach which will enable one to measure the thickness as well as other physical properties of underlayers to a high degree of accuracy. This is particularly important when non-destructive measurement of the underlayer is required and/or the underlayer changes its properties when examined directly, i.e., without the protection afforded by a top layer. It would be very desirable to provide a non-destructive measurement method for determining underlayer thickness to a high level of accuracy. It would also be desirable if such underlayer measurement could provide further information about the top layer.